1. Field of the Invention
This invention relates to a method preparing cured organopolysiloxane compositions. More specifically, this invention relates to a method for preparing cured organopolysiloxane compositions during which bubbles are not generated during curing, and which provides a cured product having an excellent surface gloss and surface smoothness.
2. Description of the Prior Art
It is known that organopolysiloxane compositions principally constituted of organopolysiloxane having silicon bonded alkenyl radicals and organohydrogenpolysiloxane having silicon bonded hydrogen atoms can be cured to form rubbery cured silicone materials by an addition reaction conducted in the presence of a noble metal from Group IV of the Periodic Table, for example, platinum, rhodium, palladium or compounds of these metals. Furthermore, it is also known that such organopolysiloxane compositions can be liquid and that, when injected or transferred into various metal molds and heated under elevated pressure, these liquid compositions are rapidly cured to silicone rubber products with an excellent mechanical strength.
U.S. Pat. No. 4,162,243, which issued to Lee et al on June 24, 1979, describes curable liquid organopolysiloxane compositions comprising a vinyl-containing polydiorganosiloxane of specified molecular weight distribution, an organohydrogenpolysiloxane and a platinum hydrosilation catalyst. In accordance with the examples of this patent the ingredients of the curable composition are blended together at ambient temperature and the compositions are cured at temperatures above 100 degrees C.
Many of the methods for preparing and curing organopolysiloxane compositions that are described in the prior art include use of an optional or required catalyst inhibitor for retarding the rate of the addition reaction at ambient temperature.
These prior art compositions suffer from several problems as a consequence of the addition of a small or extremely small quantity of a catalyst inhibitor to the aforementioned starting materials, followed by mixing of the resultant compositions at room temperature or higher temperatures, and curing of the compositions at temperatures at of 120 degrees C. or above. These problems result from air or volatile components that become trapped within the organopolysiloxane composition or metal mold. This entrapped material undergoes thermal expansion upon heating and the cured product in many cases contains extremely small bubbles in the interior or at the surface. As a result, a uniform cured silicone product with excellent physical properties often cannot be produced.
In addition to containing bubbles, the organopolysiloxane composition itself undergoes thermal hysteresis due to its thermal expansion generating irregularities on the surface of the cured product and thus preventing the production of a cured silicone product with a smooth and glossy surface.
Various methods were examined by the present inventors in order to find a method for curing organopolysiloxane compositions which would not suffer from these problems. The invention was developed as a consequence of the finding that the aforementioned problems could be substantially eliminated by mixing and curing the ingredients of a curable organopolysiloxane composition under specific temperature conditions.
An object of the present invention is to provide a method for preparing cured organopolysiloxane compositions in which bubbles are not generated and which produces a cured silicone elastomer product with excellent surface gloss and surface smoothness.